Electric cable



' July 23, 1940. Q J BEAVER EVAL 2,208,832

ELECTRIC CABLE Filed May 2, 1939 Inventor ifm-aww* md,

A Attorney ability that breakdown Patented July 23, i949 man:l @Aarts Charles .lames Beaver, Bowden,

and Edward Leslie Davey, 'li'imperley, England, assiglnors t0 W. T. Glover & Company ited, Manchester, Lancashire, England, a British com- Dany Application May 2, i939, Serial No. 271,252 In Great Britain May 3l, 1938 l Claim.

In high voltage electric cables for the purpose of reducing the local stress in the dielectric adjacent to the surface of the conductor, due to the variation of the radius of curvature of the surface of a stranded conductor, a wrapping of metallised paper has been applied to the conductor with the metallised surface in contact therewith. The invention is concerned with an improved arrangement of this kind.

It is known that there is a much greater probof a non-homogeneous -dielectric will be initiated -by a discharge in a space bounded on one side by a conductive surface than in a space bounded on all sides by a non-conductive surface. If metallised paper be wrapped helically round a conductor, with the metallic surface in contact with the conductor and with a small overlap of adjacent turns, there is not complete prevention of the existence of spaces having conducting material on one side only. The spaces between the conductor and the metallic layer have conducting surfaces on both sides, but the edge of the metallic layer, where it overlaps the side of the adjacent turn of metallised paper, lies in face of which is formed of insulating material, usually impregnated paper. There is, accordingly, in this space the possibility of the initiation of breakdown.

by the innermost layer 80 In the improved arrangement, in accordance over this overlapping edge of with the lpresent invention, this possibility is avoided by terminating the metallisation of the paper at a short distance from the edge of the paper which overlaps the adjacent turns. By thisl means the edge of the metallic layer is shut in by the narrow width of paper lyingf between it and the edge of the strip. Accordingly, there is no exposed conducting material in the space where the layer of vinsulating material passes metallised paper. The invention is further described hereinafter,

by way of example, with reference to the accompanying drawing wherein:

Figurel 1 vrepresents a longitudinal section of a single-core high voltage cable incorporating the improved construction;

Figure 2 represents diagrammatically upon an increased scale a section of a portion of such a cable representing the relative disposition of theA r .ccmponent parts adjacent the conductor sur- '='face and illustrating the form of metallisation l 'which has heretofore been generally adopted;

and

Figure 3 is a view similar to Figure 2 indicata space, the outer surenclosed in a body of in l0 As is shown diagrammatically in Figure 2 the metallised layer comprises a metallic surface l formed on one side of astrip 5 of insulating paper is helically applied to the conductor so that these gaps 8 and the overlapping parts 6 of the metallised layer are not coincident. Consequently, where the first layer of insulation covers those there is formed a space 9 surface is set back and isolated from the space 9. Thus there is no exposed conducting material in this space 9 and the undesirable electric discharge does not pccur.

It will be understood that to obtain the benefit of the invention the metallised paper must not be perforated, since the edges of the metallic layer would be exposed in the cavities thus formed and the undesirable electric discharge would occur thereat.

Where the metallisation of the paper is effected by spraying, the

provision of an unmetallised 55 part along the edge of the strip can be convenpaper overlapping and the metallic surface teriently effected by the use of a shield. minating short of the overlapping edge of the What we claim as our invention is; paper, leaving a non-metallised border between -A high voltage electric cable i'n which a. helical the said overlapping edge and the termination 0f 5 wrapping of paper strip metallised on one side the metallic Surfaceencloses the conductor with the metallic surface of the metallised-paper in contact therewith, CHART-IES JAMES BEAVER the 'adjacent turns of the helix of metallised EDWARD LESLIE DAVEY. 

